Organic light-emitting devices (OLED) are generally composed of two electrodes (an anode and a cathode) and at least one organic layer located between these electrodes. When voltage is applied between the two electrodes of the organic light-emitting device, holes and electrons are injected into the organic layer from the anode and cathode, respectively, and are recombined in the organic layer to form excitons. In turn, when these excitons decay to their ground state, photons corresponding to the energy difference are emitted. By this principle, the organic light-emitting devices generate visible ray, and they are used in the fabrication of information display devices and illumination devices.
The organic light-emitting devices are classified into three types: a bottom emission type in which light produced in the organic layer is emitted in the direction of a substrate; a top emission type in which the light is emitted in direction opposite the substrate; and a both-side emission type in which the light is emitted in both the direction of the substrate and the direction opposite the substrate.
In passive matrix organic light-emitting device (PMOLED) displays, an anode and a cathode perpendicularly cross each other, and the area of the crossing point acts as a pixel. Thus, the bottom emission and top emission types have no great difference in effective display area ratios (aperture ratios).
However, active matrix organic light-emitting device (AMOLED) displays include thin-film transistors (TFTs) as switching devices for driving the respective pixels. Because the fabrication of these TFTs generally requires a high-temperature process (at least several hundred ° C.), a TFT array required for the driving of organic light-emitting devices is formed on a glass substrate before the deposition of electrodes and organic layers. In this regard, the glass substrate having the TFT array formed thereon is defined as a backplane. When the active matrix organic light-emitting device displays having this backplane are fabricated to have the bottom emission structure, a portion of light emitted toward the substrate is blocked by the TFT array, resulting in a reduction in the effective display aperture ratio. This problem becomes more severe when pluralities of TFTs are given to one pixel in order to fabricate more elaborate displays. For this reason, the active matrix organic light-emitting devices need to be fabricated to have the top emission structure.
In the top emission type or both-side emission type organic light-emitting devices, an electrode located on the opposite side of the substrate without making contact with the substrate must be transparent in the visible ray region. In the organic light-emitting devices, a conductive oxide film made of, for example, indium zinc oxide (IZO) or indium tin oxide (ITO), is used as the transparent electrode. However, this conductive oxide film has a very high work function of generally more than 4.5 eV. For this reason, if the cathode is made of this oxide film, the injection of electrons from the cathode into the organic layer becomes difficult, resulting in a great increase in the operating voltage of the organic light-emitting devices and deteriorations in important device characteristics, such as light emission efficiency. The top emission or both-side emission type organic light-emitting devices need to be fabricated to have the so-called “inverted structure” formed by the sequential lamination of the substrate, the cathode, the organic layer and the anode.
Furthermore, if an a-Si thin-film transistor is used in the active matrix organic light-emitting device, the a-Si TFT has a structure where source and drain junctions are doped with n-type impurities because the a-Si TFT has a physical property such that the main charge carriers are electrons. Thus, in the case of fabricating the active matrix organic light-emitting device with the a-Si TFT, it is preferable in terms of charge injection and process simplification that the active matrix organic light-emitting device is fabricated to have the so-called “inverted structure” by forming the cathode of the organic light-emitting device on the source junction or drain junction of the a-Si TFT formed on the substrate, and then, sequentially forming the organic layer and the anode made of conductive oxide, such as ITO or IZO.
In a process of fabricating the organic light-emitting device with the above-described inverted structure, if the electrode located on the organic layer is formed of a transparent conductive oxide film, such as IZO or ITO, by the use of resistive heating evaporation, the resistive heating evaporation will cause the collapse of the inherent chemical composition ratio of the oxide due to, for example, thermal decomposition during a thermal evaporation procedure. This will result in the loss of characteristics, such as electrical conductivity and visible ray permeability. For this reason, the resistive heating evaporation cannot be used in the deposition of the conductive oxide film, and in most cases, techniques, such as plasma sputtering, are now used.
However, if the electrode is formed on the organic layer by techniques such as sputtering, the organic layer can be damaged due to, for example, electrically charged particles present in plasma used in the sputtering process. Furthermore, the kinetic energy of atoms, which reach the organic layer and form an electrode on the organic layer in the sputtering process, is several tens to several thousands of eV, which is much higher than the kinetic energy of atoms (generally less than 1 eV) in the resistive heating evaporation. Thus, the physical properties of the organic layer can be deteriorated by particle bombardment on the organic layer, resulting in deterioration of electron or hole injection and transport characteristics and light emission characteristics. Particularly, organic materials consisting mainly of covalent bonds of C and H, and thin films made of these materials, are generally very weak against plasma during a sputtering process, compared to inorganic semiconductor materials (e.g., Si, Ge, GaAs, etc.) and, once damaged, the organic materials cannot be returned to their original state.
Thus, in order to fabricate good organic light-emitting devices, damage to the organic layer, which can occur when forming an electrode on the organic layer by a technique, such as sputtering must be minimized or eliminated.
To avoid damage to the organic layer, which can occur when forming an electrode on the organic layer, for example, by sputtering, methods for controlling the rate of thin-film formation are used. For instance, in one method, RF power or DC voltage in an RF or DC sputtering process can be lowered to reduce the number and mean kinetic energy of atoms incident from a sputtering target onto the substrate of the organic light-emitting device, thus reducing sputtering damage to the organic layer.
In another method for preventing sputtering damage to the organic layer, the distance between the sputtering target and the substrate of the organic light-emitting device can be increased to enhance the opportunity of the collisions between atoms, incident to the substrate of the organic light-emitting device from a sputtering target, and sputtering gases (e.g., Ar), thus intentionally reducing the kinetic energy of the atoms.
However, as most of the above-described methods result in a very low deposition rate, the processing time of the sputtering step becomes very long, resulting in a significant reduction in productivity throughout a batch process for fabricating the organic light-emitting device. Furthermore, even in an instance when the sputtering process has a low deposition rate as described above, the possibility of particles having high kinetic energy reaching the surface of the organic layer still exists, and thus, it is difficult to effectively prevent sputtering damage to the organic layer.
“Transparent organic light emitting devices,” Applied Physics Letters, May 1996, Volume 68, p. 2606, describes a method of forming an anode and organic layers on a substrate, and then forming a thin layer of mixed metal film of Mg:Ag having excellent electron injection performance thereon, and lastly, forming a cathode using ITO by sputtering deposition thereon. The structure of the organic light-emitting device described in this article is illustrated in FIG. 1. However, the Mg:Ag metal film has shortcomings in that the metal film is lower in visible ray permeability than ITO or IZO and also its process control is somewhat complicated.
“A metal-free cathode for organic semiconductor devices,” Applied Physics Letters, Volume 72, April 1998, p. 2138, describes an organic light-emitting device having a structure formed by the sequential lamination of a substrate, a anode, an organic layer and a cathode, where a CuPc layer, relatively resistant to sputtering, is deposited between the organic layer and the cathode in order to prevent sputtering damage to the organic layer, which is caused by the deposition of the cathode. FIG. 2 illustrates the structure of the organic light-emitting device described in the article.
However, while CuPc is generally used to form a hole injection layer, in the above literature, CuPc serves as an electron injection layer in a state damaged by sputtering, between the organic layer and the cathode in the organic light-emitting device with a structure formed by the sequential lamination of the substrate, the anode, the organic layer and the cathode. This deteriorates device characteristics, such as the charge injection characteristic and electric current efficiency of the organic light-emitting device. Furthermore, CuPc has large light absorption in the visible ray region, and thus, increasing the thickness of the CuPc film leads to rapid deterioration of the device performance.
“Interface engineering in preparation of organic surface emitting diodes”, Applied Physics Letters, Volume 74, May 1999, p. 3209, describes an attempt to improve the low electron injection characteristic of the CuPc layer by depositing a second electron transport layer (e.g., Li thin film) between an electron transport layer and the CuPc layer. FIG. 3 illustrates the structure of the organic light-emitting device described in this literature. However, this method for preventing sputtering damage has problems in that an additional thin metallic film is required and process control also becomes difficult.
Accordingly, there is a need for the development of technology to prevent the organic layer from being damaged when forming the anode in the organic light-emitting device with the above-described inverted structure.
Meanwhile, an electron injection characteristic from a cathode to an electron transport layer in a regular organic light-emitting device, is improved by depositing a thin LiF layer, which helps the injection of electrons, between the electron transport layer and the cathode. However, the electron injection characteristic is improved only when the method is used in a device in which the cathode is used as a top contact electrode, while the electron injection characteristic is very poor when the method is used in a device having an inverted structure in which the cathode is used as a bottom contact electrode.
“An effective cathode structure for inverted top-emitting organic light-emitting device,” Applied Physics Letters, Volume 85, September 2004, p. 2469, describes an attempt to improve the electron injection characteristic through a structure having a very thin Alq3-LiF—Al layer between a cathode and an electron transport layer. However, the structure has a disadvantage that the fabricating process is very complicated. In addition, “Efficient bottom cathodes for organic light-emitting device,” Applied Physics Letters, Volume 85, August 2004, p. 837, describes an attempt to improve the electron injection characteristic by depositing a thin Al layer between a metal-halide layer (NaF, CsF, KF) and an electron transport layer. However, the method also has a problem in the process because a new layer must be used.
Accordingly, in an organic light-emitting device having an inverted structure, a method to improve the electron injection characteristic and to simplify the process for fabricating a device is required.